This invention relates generally to modified clay sorbents, and more particularly to modified clay sorbents useful for removing trace pollutants from industrial effluent streams.
There is a clear, generally known need to reduce the level of pollution in the aquatic environment. A significant first step in effecting reduction of the level of such pollution would be achieved by removing trace levels of organic contaminants, such as polychlorinated biphenyls (PCBs), polychlorinated dioxins (PCDDs), and polychlorinated dibenzofurans (PCDFs). Such trace organic contaminants are found in industrial effluent water as undesirable by-products of chemical manufacturing. For example, 2,3,7,8-Tetrachlorodioxin (2,3,7,8-TCDD) is formed during the production of 2,4,6-Trichlorophenol, which is an ingredient of many pesticides. Even at extremely low concentrations, 2,3,7,8-TCDD is the most toxic isomer of dioxin and is generally referred to as "Dioxin." Polychlorinated dioxins are commonly found in paper mill effluents and polyaromatic hydrocarbons contaminate coal conversion waste water streams.
A known, highly advantageous technique for removing trace amounts of organic pollutants in very large volumes of waste water, utilizes a sorbent to which the contaminant is adsorbed. The most commonly used adsorbent today is activated carbon, the production of which utilizes a wide variety of carbonaceous starting materials, such as anthracite and bituminous coal, carbonized shells, peat, etc. The various known methods of activating charcoal can be grouped into two categories. The first category includes chemical activation wherein the carbonaceous materials are impregnated with an activating agent and then pyrolyzed. The second category comprises heat treatment processes wherein chars are heated to temperatures between 350.degree. and 1000.degree. C. in the presence of CO.sub.2, N.sub.2, O.sub.2, HCl, Cl.sub.2, H.sub.2 O and other gases. A portion of the char is burned as the surface area and "activity" of the carbon increases. Modern manufacturing techniques, which include careful monitoring of the activation parameters, yield activated products having high surface areas in a wide range of uniform particle sizes.
In addition to activated charcoal, natural soils and sediments containing various amounts of bound organic carbon or synthetic "zeolite-like" sorbents with specific affinity for rigid, planar aromatic molecules like dioxin have been considered. All of the known systems have significant problems, however, which render them substantially less than ideal for the purpose of removing trace organics from large quantities of water. Activated charcoal and synthetic sorbents are very expensive and therefore do not provide an economical solution to the problem. Natural soils and sediments are less expensive than the activated charcoal and synthetic sorbents, but are not uniform, and in fact exhibit too much variation in binding affinity to be used in large scale technology.
In addition to the systems discussed hereinabove for the disposal of organic pollutants, such as Dioxin, a variety of other methods are known. These include: photolytic dechlorination, microbial degradation, and thermal decomposition. It is a problem with all three of these processes that they are ineffective when the polluting compound is present in water at extremely low concentrations, illustratively on the order of pptr to ppq. To date, however, no microorganism has yet been developed which is capable of degrading Dioxin. The process of thermal decomposition requires operating temperatures in excess of 1000.degree. C., and the technology required to implement this process is still in a developmental stage.
Swelling clays, generically designated as smectites, have been used widely in the prior art as catalysts, catalyst supports, molecular sieves, adsorbers, and absorbers. The utility of these clays for the stated purposes is derived from the physical properties of the clay itself. Smectites, such as montmorillonite, have a layered lattice structure in which two-dimensional oxyanions are separated by layers of hydrated cations. The layered structure enables intercalation of layers of a different character between the sheets of the clay structure.
"Pillared" clays have been developed wherein intercalated thermally stable cations act as props, or pillars, to support the silicate layers of the clay in the absence of a swelling solvent. In one particular example, the clay is treated with hydroxy aluminum polymers or oligomers in solution, and subsequently the clay is dried and calcined to produce supporting pillars between clay layers. Such pillars leave pores having a rectangular opening configuration, framed by the pillars and the clay layers. The pillar size, or spacing, and hence, the pore size of the clays can be adjusted so as to permit the making of suitable catalysts, catalyst supports, molecular sieves, etc., for various purposes, particularly in the petroleum processing field. The "pillared" clays, however, are not usable as a sorbent for industrial pollutants in an aqueous effluent stream. It is to be remembered that the term "adsorption" generally refers to a first step in catalysis and may not necessarily be applicable to the mechanism of a process for removing pollutants from industrial effluents. As is known, a catalyst will adsorb and release. Effluent treatment, however, requires that the composition adsorb the pollutant and retain same.
Organoclays have been used to remove organic contaminants from waste water. Several such examples are disclosed in U.S. Pat. No. 4,549,966 issued to Beall on October 29, 1985. Conventional organoclays have cationic surfactant ions localized in the interlayer space between clay lamellae. Organoclays are typically synthesized by cation-exchange of the cation of an ion exchange form of clay, such as Na.sup.+ -montmorillonite, with the organic surfactant cation.
A specific example of an organoclay is cetylpyridinium chloride-montmorillonite which is formed by ion-exchanging Na.sup.+ -montmorillonite with cetylpyridinium chloride, a monovalent surfactant. Since montmorillonite is an expandable clay, the cetylpyridinium ion (cationic surfactant) will be localized in the interlayer space between clay lamellae through strong and favorable electrostatic interaction between the negatively-charged clay surface and the cationic head group of the cetylpyridinium ion. The hydrocarbon tail of the surfactant ion will point away from the clay surface, thus providing an organic carbon surface layer for the sorption of toxic wastes. Reference to FIG. 1 shows an idealized schematic representation of a prior art organoclay.
Another type of modified clay sorbent which has been utilized to remove traces of organic contaminants from aqueous effluents is more completely described in U.S. Pat. No. 4,740,488 issued to Fogler, et al. on April 26, 1988. This clay is an example of a new class of two-dimensional "zeolite-like" clays which have an increased surface area and an extensive micropore structure. The term "inorganoclay" is used herein to designate this type of modified clay sorbent. Reference to FIG. 1 shows an idealized schematic representation of the structure of an inorganoclay. The microporosity of these sorbents in combination with the hydrophilic nature of its surface results in an extremely effective sorbent for the specific adsorption of highly hydrophobic, toxic organics. A specific example of such an inorganoclay is the hydroxy aluminum montmorillonite clay described in the aforementioned U.S. Pat. No. 4,740,488.
It is, therefore, an object of this invention to provide an inexpensive and effective system for removing organic contaminants from an aquatic environment.
It is another object of this invention to provide a system for removing organic contaminants present in an aqueous environment in trace concentrations on the order of between ppm and ppq.
It is also an object of this invention to provide a system for removing organic contaminants from an industrial waste water streams, the contaminants including, inter alia, dioxins, polyaromatics, PCBs, PCDFs, and PCDDs.
It is a further object of this invention to provide a sorbent material having high sorptive capacity and binding affinity for organic pollutants.
It is additionally an object of this invention to provide a sorbent material having a greater sorptive capacity for organic contaminants than activated carbon, the current industry standard, and which is more economical to produce than activated carbon.
It is still another object of this invention to provide a sorbent material which is inexpensive to use, due to, inter alia, low cost starting materials and high sorptive capacity so that smaller quantities of sorbent are required.
It is yet a further object of this invention to provide a process for removing trace organic contaminants which can be used simply and with existing technology and is inexpensive to implement.
It is yet an additional object of this invention to provide a sorbent material which can easily, and safely, be disposed of after it has been used to remove trace organic pollutants.